1) Field of the Invention
The present invention relates to a rust-preventive lubricant composition for a zinc-plated steel material. More particularly, the present invention relates to a rust-preventive lubricant composition providing an excellent lubrication of a zinc-plated steel material subjected to a press forming process, and a satisfactory removability thereof after the press-forming process.
2) Description of the Related Arts
Conventional lubricants used in the press-forming of a zinc-plated steel material comprise a press oil and a rust-preventive oil.
Conventional press oils comprise a base oil component composed of an animal or vegetable oil, mineral oil or a synthetic oil, and an additive component comprising an extreme pressure additive and a rust-inhibiting additive. In general, when the conventional press oil is applied to a steel material, the resultant press oil-applied steel material exhibits an enhanced press-formability, but the corrosion resistance of the resultant press-formed steel material and the removability of the press oil from the steel material are unsatisfactory.
The conventional rust-preventive oils comprise a base oil component composed of a mineral oil and a rust-preventive additive component mixed into the base oil component. When the conventional rust-preventive oils are applied to a steel material, the resultant steel material exhibits an unsatisfactory press-formability, although the corrosion resistance of the resultant steel material and the removability of the rust-preventive oils on the resultant steel material are satisfactory.
A steel material treated with a lubricant composition for plastic processing and able to be easily degreased is disclosed, for example, in Japanese Examined Patent Publication No. 53-37882.
In recent years, the degreasing temperature of the press-formed steel material has been lowered to a level of 40.degree. C. to 45.degree. C. Accordingly, when the lubricant composition of the Japanese publication is applied, the resultant steel strip exhibits an unsatisfactory corrosion resistance and is not easily degreased, although the lubrication thereof is enhanced. Namely, the lubricant composition is disadvantageous in that, after the press-forming process, the coated lubricant composition layer remaining on the steel material surface has an unsatisfactory removability.
The conventional lubricants for press-forming processes have been widely applied to cold rolled steel strips, and when the conventional lubricants per se are applied to zinc-plated steel materials, various problems arise. For example, when a zinc-plated steel material is treated with the conventional lubricant composition for a press forming process, and then press-formed, the coated zinc layer on the steel material is deformed or abraded by the tool. In this press forming process, the conventional lubricant composition tends to promote a powdering phenomenon in which a portion of the coated zinc layer on the steel material is broken up and powdered and/or a flaking phenomenon in which a portion of the coated zinc layer is peeled from the steel material surface and flaked.
Also, the conventional lubricant composition promotes an undesirable formation of white rust, which is peculiar to the zinc-plated steel material, and the lubrication by the lubricant composition is affected by the resultant white rust.
In recent years, the use of zinc-plated steel material in the car industry and home electric appliance industry has increased, and thus an improvement in the corrosion resistance of the zinc-plated steel material to be exported is strongly demanded. Further, the zinc-plated steel material must have an enhanced chemical conversion property after press-forming, and a corrosion resistance after paint coating, and the lubricant to be applied to the zinc-plated steel material must provide an improved removability from the press-formed steel material.
Nevertheless, it is difficult to find a satisfactory conventional lubricant meeting the above-mentioned requirements.
Therefore, there is a strong demand for a rust-preventive lubricant useful for a press-forming process of a zinc-plated steel material and providing an excellent lubrication, a superior corrosion resistance, and a satisfactory removability.